MEMS devices are more and more used in microelectronics, the market of these devices continually increasing every year. As an example of a MEMS device we shall use, for the purpose of this application a MEMS microphone. The MEMS microphone market is approximately doubling every year.
FIG. 1 depicts a typical DC-DC converter adapted to supply a MEMS microphone and in general to a MEMS device, which is denoted MEMS in FIG. 1. The converter receives an input direct current (DC) voltage Vs and converts it into two higher level DC voltages V1 and V2. The voltage V1 is transmitted to the MEMS device via the pairs of diodes D1,D2. The V2 is transmitted to the middle node of two capacitors in series via diodes D3,D4. The diodes are connected in the well-known back to front connection or anti-parallel. Sw1 switch which intermittently connect the said voltage to the MEMS device. Sw2 switch which intermittently connect the said voltage to the middle node of two capacitors in serives. In parallel with the MEMS device there is a series connection of two equal-value capacitors C1 and C2. The capacitors determine a mid-point voltage Vme and further smooth the voltages V1 and V2.
There are some issues related to this DC-DC converter and we shall outline them assuming that the output voltage of the converter is 10.77 volt and the MEMS device is a MEMS microphone.
Problem 1. The 10.7V DC-DC converter needs also a load capacitor of 45 pF for MEMS Microphone. So, C1=C2=90 pF are used. The voltage at the middle node between two series capacitors should be maintained between maximum of 5.5V and minimum 5.3V (which is about half of the output voltage of 10.7V). If this middle node voltage (MNV) goes above 5.5V (e.g. 6V) then the capacitor C2 can be damaged and 10.7V DC-DC converter becomes useless.
Problem 2. During startup the middle node voltage may go beyond 5.5V. Again this will damage the capacitor C2 and the 10.77V DC-DC converter will not work properly anymore. This is a newly identified problem, which was not previously reported.
Problem 3. The ripple at output voltage V1=10.77V does not meet specifications of below 10 uV.